High-frequency amplifying circuits that include an amplifier based on linear amplification with nonlinear components (LINC) are known as units for implementing a high-efficiency liner amplifier.
FIG. 1 is a diagram illustrating an example of a LINC-based amplifier. In the LINC-based amplifier, a LINC signal generating unit separates an input modulation signal Sin(t) into a pair of phase modulation signals Sc1(t) and Sc2(t) and outputs the pair of phase modulation signals Sc1(t) and Sc2(t). The phase difference between the phase modulation signals Sc1(t) and Sc2(t) corresponds to the amplitude of the input modulation signal Sin(t). For example, the input modulation signal Sin(t) is a modulation signal exhibiting amplitude modulation and phase modulation (angular modulation) and the pair of phase modulation signals Sc1(t) and Sc2(t) are constant-envelope, constant-amplitude phase modulation signals. Here, the input modulation signal Sin(t) and the pair of the phase modulation signals Sc1(t) and Sc2(t) may be baseband signals or intermediate frequency (IF) signals. The LINC signal generating unit outputs the pair of phase modulation signals Sc1(t) and Sc2(t) as digital signals.
Here, the signals Sin(t), Sc1(t), and Sc2(t) are represented, for example, by Expression 1.
                                          Sin            ⁡                          (              t              )                                =                                                    a                ⁡                                  (                  t                  )                                            ·              cos                        ⁢                                                  ⁢                          θ              ⁡                              (                t                )                                                    ⁢                                  ⁢                              Sc            ⁢                                                  ⁢            1            ⁢                          (              t              )                                =                                    a              max                        ·                          cos              ⁡                              (                                                      θ                    ⁡                                          (                      t                      )                                                        +                                      ψ                    ⁡                                          (                      t                      )                                                                      )                                                    ⁢                                  ⁢                              Sc            ⁢                                                  ⁢            2            ⁢                          (              t              )                                =                                    a              max                        ·                          cos              ⁡                              (                                                      θ                    ⁡                                          (                      t                      )                                                        -                                      ψ                    ⁡                                          (                      t                      )                                                                      )                                                    ⁢                                  ⁢                              ψ            ⁡                          (              t              )                                =                                    cos                              -                1                                      ⁡                          (                                                a                  ⁡                                      (                    t                    )                                                                    2                  ·                                      a                    max                                                              )                                                          [                  Expression          ⁢                                          ⁢          1                ]            
In Expression 1, “a(t)” represents the amplitude component of the input modulation signal Sin(t), “θ(t)” represents the phase component of the input modulation signal Sin(t). Phase modulation is provided so that a phase difference of 2×ψ(t), which corresponds to the amplitude a(t), is generated. Furthermore, “amax” represents the maximum value of the amplitude a(t) and is a constant. The signals Sc1(t) and Sc2(t) are constant envelope signals. That is, the amplitude of the signals Sc1(t) and Sc2(t) is fixed.
The signal Sc1(t), one of the pair of phase modulation signals output from the LINC signal generating unit, is converted from a digital signal into an analog signal by a digital-to-analog converter (DAC). Furthermore, when the converted analog signal passes through a low-pass filter, a component corresponding to a frequency band of the phase modulation signal Sc1(t) is extracted, and the other frequency components are suppressed. Similarly, the signal Sc2(t), the other of the pair of phase modulation signals, is converted from a digital signal to an analog signal by a DAC. Furthermore, when the converted analog signal passes through a low-pass filter, a component corresponding to the frequency band of the phase modulation signal Sc2(t) is extracted, and other frequency components are suppressed.
In the LINC-based amplifier, a quadrature modulator performs quadrature modulation on the phase modulation signal Sc1(t) which has passed through the corresponding low-pass filter. A frequency converter generates, using a high-frequency signal (oscillation signal) output from an oscillator, a signal S1(t), which is one of a pair of high-frequency signals that are radio-frequency (RF) signals, and outputs the generated high-frequency signal S1(t). Similarly, a quadrature modulator performs quadrature modulation on the phase modulation signal Sc2(t) which has passed through the corresponding low-pass filter. A frequency converter generates, using a high-frequency signal output from an oscillator, a signal S2(t), which is the other of the pair of high-frequency signals that are RF signals, and outputs the generated high-frequency signal S2(t).
The high-frequency signals S1(t) and S2(t) are represented by Expression 2, where “fc” represents a radio frequency (the frequency of the oscillator).S1(t)=amax·cos(2π·fc·t+θ(t)+ψ(t))S2(t)=amax·cos(2π·fc·t+θ(t)−ψ(t))  [Expression 2]
A pair of amplifiers include two amplifiers arranged in parallel to each other. The gain and phase characteristics of the two amplifiers are substantially the same. The amplifiers each amplify a high-frequency signal output from the corresponding frequency converter. A portion from the DAC to the amplifier (amplifier A or B) inclusive is also called one branch.
A combiner combines the pair of high-frequency signals amplified by the pair of amplifiers together, and outputs the combined signal as a high-frequency signal Sout(t). The signal Sout(t) output from the combiner is represented by Expression 3, where “G” represents the gain of the amplifiers.
                                                                        Sout                ⁡                                  (                  t                  )                                            =                            ⁢                                                G                  ·                                      a                    max                                    ·                                      cos                    ⁡                                          (                                                                        2                          ⁢                                                                                                          ⁢                                                      π                            ·                            fc                            ·                            t                                                                          +                                                  θ                          ⁡                                                      (                            t                            )                                                                          +                                                  ψ                          ⁡                                                      (                            t                            )                                                                          +                        ϕ                                            )                                                                      +                                                                                                      ⁢                              G                ·                                  a                  max                                ·                                  cos                  ⁡                                      (                                                                  2                        ⁢                                                  π                          ·                          fc                          ·                          t                                                                    +                                              θ                        ⁡                                                  (                          t                          )                                                                    -                                              ψ                        ⁡                                                  (                          t                          )                                                                    +                      ϕ                                        )                                                                                                                          =                            ⁢                              2                ⁢                                  G                  ·                                      a                    max                                    ·                                      cos                    ⁡                                          (                                                                        2                          ⁢                                                      π                            ·                            fc                            ·                            t                                                                          +                                                  θ                          ⁡                                                      (                            t                            )                                                                          +                        ϕ                                            )                                                                      ⁢                                  cos                  ⁡                                      (                                          ψ                      ⁡                                              (                        t                        )                                                              )                                                                                                                          =                            ⁢                              2                ⁢                                  G                  ·                                      a                    ⁡                                          (                      t                      )                                                        ·                                      cos                    ⁡                                          (                                                                        2                          ⁢                                                      π                            ·                            fc                            ·                            t                                                                          +                                                  θ                          ⁡                                                      (                            t                            )                                                                          +                        ϕ                                            )                                                                                                                              [                  Expression          ⁢                                          ⁢          3                ]            
In Expression 3, “φ” represents the transmission phase of the pair of high-frequency signals S1(t) and S2(t).
Related arts are disclosed in, for example, Japanese National Publication of International Patent Application No. 2009-533947 and Japanese Laid-open Patent Publication Nos. 2003-298361, 2003-152464, 5-37263, 9-74320, and 2006-33988.